Can anyone explain what a power tube does inside an amplifier, eg kt88.

"I guess current flows in...." Works for me. :-)

If you want a tutorial or an actual answer I think there are better places to ask this kind of question. 

Signal enlarge through the air in the power by the electrical pressure.

a tube is connected first to the filament supply that heats up the cathode so it can emit electrons inside vacuum.

A small offset voltage can be applied to the grid and plates of the tube.

KT88 is a penthode tube connected usually as a tetrode in the tube amps with usage of an extra reflection device called plate. There are 2 plates in penthode totally giving 5 electrodes: Cathode, Anode, Grid, Plate1 and Plate2. 

An input signal is usually applied to the grid. That's where you get the MOST voltage gain and the output to the transformer is going to be between anode and ground or Cathode is in this case usually grounded. 

@emergingsoul 

You're certainly good at starting weird threads.

There are hundreds of websites explaining this in dozens of different ways.  Google it and find one that suits your mind.